In 1979 the IEEE published an article which has become a basic reference in the field of television echo (or "ghost") elimination. The article is entitled "A Tutorial On Ghost Cancellation In Television Systems" and was written by Walter Ciciora, Gary Sgrignoli and William Thomas and it is incorporated by reference herein.
Although the Ciciora article described the fundamental principles, apparatus and algorithms applicable to ghost cancellation, the state of the art has only recently progressed to the point of providing practical ways to implement and improve these basic concepts.
There are two main steps to the echo cancellation process. First the characteristics of the communications channel (which include the echo artifacts, if any) must be determined at the receiver. Once the channel characteristics are calculated, filters are used to implement the inverse channel characteristics to substantially eliminate the echoes. The present invention relates to an apparatus and an improved ghost cancellation reference signal, for identifying the characteristics of a communication channel.
Communication engineering continually must deal with the problem of restoring a signal which has been altered by the communication path over which the signal was transmitted. Signal restoration often can be achieved if the communication path is fully characterized, at least as to those parameters which contribute to the signal alteration. Thus, a frequently essential component of the signal restoration problem is that of identifying the characteristics of the communication path or channel.
A straight forward approach to the channel identification problem is to transmit a ghost cancellation reference signal (GCR) having a known characteristic, over the channel, and to receive the transmitted signal after it has passed through the channel. The originally transmitted signal is compared with the received signal, and a model of the channel characteristics is developed based on the comparison.
The Japanese Broadcasting Technology Association (BTA) has adopted a GCR signal that is the time integral of a windowed sin x / x pulse (sinc) which is transmitted on line 18 of the vertical blanking interval (VBI) of a television signal. Although the BTA GCR signal has the necessary flat bandwidth in the frequency domain, its energy is relatively low. The BTA GCR signal therefore may be suboptimal since its low energy limits its performance under high noise conditions. Additional processing time is needed to compensate for the noise present in the channel which increases the time it takes for the echo cancellation system to calculate the channel characteristics when conditions in the channel change. The BTA GCR signal has a fixed time interval which cannot be changed without effecting its frequency spectrum characteristics. This limits the possible applications for which the BTA GCR signal can be used. The time interval for an NTSC television system, for example, would be .gtoreq.52.5 .mu.s.
Other GCR signals have been proposed which have a higher energy level than the BTA signal. These signals, are cyclic in nature however, and therefore not effective for detecting pre and post-echoes present in the channel.